This invention relates to fiber-reinforced driveshafts, particularly--but not exclusively--cardan shafts and analogous drive shafts for motor vehicles.
More especially, the invention relates to a drive shaft of fiber-reinforced synthetic plastic material, and to a method of making the same.
Fiber-reinforced synthetic plastic drive shafts are not entirely new. German Published Application DE-OS No. 2,851,292 discloses a cardan-shaft of this type which has the advantage of exhibiting better vibratory characteristics than comparable metal cardan shafts, and of having a lower weight with a consequent improvement in performance and in fuel use. However, for each engine type a separate cardan shaft must be produced or stocked, so that the above-mentioned advantages are in part negated by the greater manufacturing and/or stock-keeping efforts required.
Another fiber-reinforced synthetic plastic cardan shaft is known from German Published Application DE-OS No. 2,851,293. There, ring-shaped metallic end pieces are inserted over the major part of their length into the ends of a fiber-reinforced tubular shaft of synthetic plastic material. The end pieces are mounted on a mandrel at a distance corresponding to the length desired for the finished shaft, the glass-fiber-reinforced synthetic plastic material is applied over the mandrel and the major portion of the length of each end piece, and is then hardened. Thereupon the shaft is pulled off the mandrel and the projecting portions of the end pieces are welded to connecting elements (such as the yoke of a cardan joint) and finally the shaft is balanced to make it run "in the round".
A problem encountered in connection with this prior-art disclosure, is that it is difficult to so fix the end pieces on the mandrel at the required spacing (without resorting to complicated devices) that the shaft can subsequently be pulled off the mandrel without damaging the same. For this reason, throw-away mandrels are used which are left in the shaft. This, however, creates another problem, in that such mandrels increase the weight and cost of the finished shaft without, however, increasing its mechanical strength. Also, the simple application of the glass-fiber reinforced synthetic plastic in form of a coating over the mandrel and the end pieces, does not (after hardening) assure the high mechanical strength--especially torsion and bending resistance--which is desirable for drive-shaft applications. To achieve this a special structure and orientation of the reinforcing fibers is needed, which is not disclosed in the reference.
Also, the end pieces are anchored in the tubular shaft only by relatively low, primarily adhesive forces. The very sensitive end faces of the tubular shaft are exposed to all ambient influences and thus subject to damage. The multiple abrupt transitions from the cross-section of the end pieces to the cross-section of the tubular shaft are a problem, especially where the shaft cross-section is reduced at the inner end of each end piece, i.e. just where the torque is transmitted between the end pieces and the shaft. This leads to locally excessive thrust which may cause a destruction of the shaft in this critical transition zone. For these reasons the known cardan shaft does not have sufficient tensile, compressive shear, bending and torsional resistance to withstand the very high mechanical loads acting upon a cardan shaft, for a sufficient length of time. Destruction may occur after only a relatively short period of use.